shaman + druid is a good combo btw, and paladin/disc is good against iron prisons.

Keep efflorence up
Keep rejuv up at the other healer and you at all times
Regrowth when it's needed and with procs.
rotate cd's iron bark and barkskin when 11 seconds left, reduces a lot of the incoming damage.
save something like ironbound fortitude, fortifying brew and such for falling ash.

We have a shaman, pala and myself druid, so would it be better to have me and shaman on Harom group and pala at the other group?

Wont HoP be missed up on Haromm group?

Pala needs to be downstairs for absorbs. HoP and/or purity is overrated anyways, it's all survivable with personals (mostly . Depending on how many melee you have in your group, you may not need multiple healers on top. (If you only have tanks as melee, just one heal it on top)